Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Short-term investments	$ 4,494	$ 26,816
Interest rate swap agreements	1,368	52
Total assets	365,937	106,124
Liabilities
Total liabilities	13,290	171,869
Money market funds
Assets
Cash equivalents	360,075	79,256
Natural gas fixed price forward contracts
Liabilities
Derivatives	11,445	15,368
Embedded derivative on 6% promissory notes
Liabilities
Derivatives		140,771
Interest rate swap agreements
Liabilities
Derivatives	1,845	5,905
Preferred stock warrants
Liabilities
Stock warrants		9,825
Level 1
Assets
Short-term investments	4,494	26,816
Interest rate swap agreements	0	0
Total assets	364,569	106,072
Liabilities
Total liabilities	0	0
Level 1 | Money market funds
Assets
Cash equivalents	360,075	79,256
Level 1 | Natural gas fixed price forward contracts
Liabilities
Derivatives	0	0
Level 1 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives		0
Level 1 | Interest rate swap agreements
Liabilities
Derivatives	0	0
Level 1 | Preferred stock warrants
Liabilities
Stock warrants		0
Level 2
Assets
Short-term investments	0	0
Interest rate swap agreements	1,368	52
Total assets	1,368	52
Liabilities
Total liabilities	1,845	5,905
Level 2 | Money market funds
Assets
Cash equivalents	0	0
Level 2 | Natural gas fixed price forward contracts
Liabilities
Derivatives	0	0
Level 2 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives		0
Level 2 | Interest rate swap agreements
Liabilities
Derivatives	1,845	5,905
Level 2 | Preferred stock warrants
Liabilities
Stock warrants		0
Level 3
Assets
Short-term investments	0	0
Interest rate swap agreements	0	0
Total assets	0	0
Liabilities
Total liabilities	11,445	165,964
Level 3 | Money market funds
Assets
Cash equivalents	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives	11,445	15,368
Level 3 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives		140,771
Level 3 | Interest rate swap agreements
Liabilities
Derivatives	0	0
Level 3 | Preferred stock warrants
Liabilities
Stock warrants	$ 0	$ 9,825